Hitherto, since resorcinol-formalin resins have been used for adhesives, plywood, glued laminated timber, surface coating agents, and the like owing to their fast curing rate and, in particular, they are used as tire adhesives and rubber hose adhesives owing to their excellent adhesive force to rubber and fiber (For example, cf. Patent Documents 1, 2, 3, and 4).
In the case that a resorcinol-formalin resin is used as an adhesive, it is required that the resorcinol-formalin resin has a sufficient flowability and, when a solvent coexists, the resin is homogeneously dissolved therein. When the flowability is focused on, it is empirically known that a sufficient flowability is obtained by reducing the composition ratio of resorcinol pentanuclear or higher nuclear bodies among the constitutive components of a polycondensate. It has been considered that the resorcinol pentanuclear or higher nuclear bodies contain a rapidly increased ratio of three-dimensional structures and hence the flowability is lost. Moreover, it is also possible to dilute the resin with an organic solvent in order to impart the flowability but the use of the organic solvent is not preferable since the use may possibly result in deteriorated working environment and lowering of adhesive force. There is a method of dispersing the resin in water by the action of an anionic surfactant without using any organic solvent to prepare an adhesive (for example, cf. Patent Document 5) but there still remains an anxiety of long-term stability of the resulting water dispersion system.
For the purpose of reducing the content of unreacted formaldehyde in a resin, there is a report wherein a resin is produced by a two-step reaction (for example, cf. Patent Document 6). It is a patent application relating to a process for producing an aldehyde resin to be used as an adhesive for water-repellant paper products such as cardboard. In Examples therein, a first-step reaction is conducted between formaldehyde and methyl ethyl ketone and then a second-step reaction is carried out between the resulting product and resorcinol, thereby the formaldehyde content of the product being reduced to 0.1%. However, the aldehyde resin is not an application of resorcinol-formalin resins.
Patent Document 1: JP-B-48-12185
Patent Document 2: JP-A-4-148920
Patent Document 3: JP-A-6-100850
Patent Document 4: JP-A-2000-178849
Patent Document 5: JP-A-57-167342
Patent Document 6: EPO498301A2
As mentioned above, it is known that a sufficient flowability is obtained in the case that the solvent is water by lowering the composition ratio of resorcinol pentanuclear or higher nuclear bodies among the constitutive components of the polycondensate. However, the case that the reaction conditions are set mild so as to reduce the composition ratio of resorcinol pentanuclear or higher nuclear bodies only results in the shift of the molecular weight distribution of the product to a low-molecular-weight side and usually, the concentration of unreacted resorcinol increases. When the concentration of resorcinol increases, there is a possibility that resorcinol is sublimated upon use as an adhesive to deteriorate working environment and further to lower adhesive force, so that the case is not preferred. When 0.6 mol of formaldehyde is reacted with 1 mol of resorcinol, about 33% by weight of unreacted resorcinol is contained after the completion of the reaction and when 0.8 mol of formaldehyde is reacted, about 20% by weight of unreacted resorcinol is contained. Therefore, in order to reduce the amount of unreacted resorcinol to 15% by weight, it has been reported that resorcinol is removed by sublimation under reduced pressure of 0.05 mmHg at 130° C. so as to reduce the content of unreacted resorcinol (for example, cf. Patent Document 7). Moreover, it has been reported that the content of unreacted resorcinol is reduced to 5.5% by using methyl isobutyl ketone as a solvent and water as an extracting agent by means of a continuous extractor (for example, cf. Patent Document 8). However, these processes are industrially disadvantageous since vacuum distillation is required after the completion of the reaction or a long-time operation is required using the continuous extractor.
To the contrary, when severer reaction conditions are adopted in order to lower the concentration of unreacted resorcinol, resorcinol polynuclear bodies of pentanuclear or higher nuclear bodies are formed in a large amount and hence the case is not preferred. Since the reaction is conducted in an aqueous system, it may be possible to use a method of adding a high concentration of a salt after the completion of the reaction to lower the solubility of the polymeric components and removing them after their precipitation. However, this method requires further incorporation of the salting-out step and hence is disadvantageous. In addition, there is a fear of lowered adhesive force owing to the remaining inorganic salt in the resorcinol-formalin resin or corrosion of substrate to be adhered attributable to the inorganic salt, so that the method is not practiced. A means for achieving no increase in the composition ratio of heptanuclear or higher nuclear bodies even in a one-stage reaction without incorporating the salting-out step is disclosed (for example, Patent Document 9). It is a method of coexisting a large amount of a salt in the reaction system in order to reduce the solubility of the resorcinol polynuclear bodies. In this method, since the resorcinol polynuclear bodies precipitated from the aqueous phase form a gummy matter, resorcinol is also incorporated into the gummy matter and, as a result, a decrease in the reaction rate is invited, which results in a prolonged reaction time. In industrial production, the formation of a gummy matter in a long-term reaction induces discontinuation of stirring, choking of liquid transferring system, and the like, so that it is difficult to adopt the method.
Patent Document 7: JP-B-54-932
Patent Document 8: JP-B-49-14550
Patent Document 9: JP-A-2003-277308